223 



Note on Wenham's Method for obtaining an Oblique View of 

 A Microscopic Object and on Marshall's Zoophyte Trough. 



By J. E. Ingpen, F.R.M.S. 



(Read June 17th, 1892.) 



Mr. Ingpen made some remarks with reference to his exhibit 

 of Diatoms and Lepisma scales, shown by Mr. Wenham's method 

 of obtaining oblique vision of surface markings. For this pur- 

 pose a slip of glass about ^-^ of an inch wide was ground and 

 polished at one end to an angle. The objects were scraped up 

 with the knife-edge, and another similar slip pressed against it 

 to recompose or neutralize the colour, the light thus entering 

 and issuing at right angles to the slip. The angle for dry 

 lenses must be less than 40"", about 35^ being suitable ; for bal- 

 samed objects 45° would be preferable. 



The objective, if of large aperture, must be adjusted for each 

 thickness of the upper prismatic edge, according to the position 

 of the object. 



The Diatoms shown were immersed in cedar oil ; the Lepisma 

 scales were dry. 



The original paper will be found in the " Monthly Micro- 

 scopical Journal," Vol. xiii. (1875), p. 156. 



Mr. Ingpen regretted that so valuable a method of observa- 

 tion should have been so little employed. He himself only re- 

 membered it somewhat recently upon ct)ming across some of the 

 prisms he had purchased in 1875. Independently of its assist- 

 ance in the solution of problems of insect-scale structure, etc., 

 its use in determining the real forms of diatoms and other 

 organisms, by viewing them in an oblique direction, was of 

 importance. Messrs. Ross and Co. had not now any of the 

 prisms on sale, but there would be no difficulty in getting them 

 made, if there were any demand for them, as Mr. Wenham had 

 given full directions for their construction. 



Mr. Ingpen also exhibited and described a zoophyte trough 



